The aim of this proposed research is to develop a clearer understanding of the ontogeny of the immune system, specifically differentiation of prothymocytes to mature T cells in fetal, adult and aged mice. To achieve this aim, prothymocytes will be isolated from 10 day gestation yolk sac, 14 day fetal liver, adult and aged mouse bone marrow, using their propensity to migrate toward supernatants produced by mouse thymus in culture. The cells thus isolated will be characterized according to known T cell markers and their morphology determined immediately following migration and following a period of culture in thymus supernatant. The colony forming potential of these isolated cells will be determined by culturing them on a chick chorioallantoic membrane (CAM) and by injection into an irradiated host. Fluorescein isothiocyanate, 3H-thymidine and the T6 chromosome will be used as markers in order to determine the source of origin of cells in CAM's, spleen colonies or invading the thymus. The presence of T cell markers on the cells in colonies will also be assessed to determine if T cell induction has occurred. Through this study the capacity of stem cells to seed the thymus and the receptivity of the thymus to seeding from early development through old age, will be determined. The basic information will be useful in developing and testing rational therapy for disease processes resulting from malformation of the immune system. It might also be of potential use for depleting human bone marrow of mature forms of lymphocytes before transplantation in order to reduce the risk of graft-versus-host reaction.